Trees (2004) 18:43–53
DOI 10.1007/s00468-003-0279-6
ORIGINAL ARTICLE
Radim Adolt · Jindrich Pavlis
Age structure and growth of Dracaena cinnabari populations on Socotra
Received: 10 January 2003 / Accepted: 28 May 2003 / Published online: 26 July 2003
Springer-Verlag 2003
Abstract Unique Dracaena cinnabari woodlands on
Socotra Island—relics of the Mio-Pliocene xerophilesclerophyllous southern Tethys Flora—were examined in
detail, especially with regard to their age structure.
Detailed statistical analyses of sets of 50 trees at four
localities were performed in order to define a model
reflecting relationships between specific growth habit and
actual age. The problematic nature of determining the age
of an individual tree or specific populations of D.
cinnabari is illustrated by three models relating to orders
of branching, frequency of fruiting, etc. which allow the
actual tree age to be calculated. Based on statistical
analyses as well as direct field observations, D. cinnabari
populations on Socotra do not regenerate to a great extent
and their age structure generally indicates overmaturity.
The unique Firmihin D. cinnabari woodland will reach
the stage of intensive disintegration within 30–77 years
with 95% probability. According to our analysis of dead
trees, it is evident that, on average, D. cinnabari in
populations at Firmihin dies after reaching 17 orders of
peripheral branches.
Keywords Age determination · Arborescent Dracaena ·
Dracaena cinnabari woodland · Socotra Island · Yemen
Introduction
Dragon’s blood, a highly prized product of the ancient
world (Milburn 1984), still makes the tree Dracaenas
relatively famous. This deep red liquid exuding from
injured bark is known in Arabia as ‘dammalachawin’ or
‘cinnabar’. Local people from Socotra still use it to cure
gastric sores, dye wool, glue and decorate pottery and
houses (Miller and Morris 1988). Although comprehensive information on chemical features of the dragon’s
R. Adolt ()) · J. Pavlis
Faculty of Forestry and Wood Technology,
Mendel University of Agriculture and Forestry (MUAF),
Zemedelska 3, 613 00 Brno, Czech Republic
e-mail: [email protected]
blood resin is available (Himmelreich et al. 1995;
Vachalkova et al. 1995), too few studies on growth
dynamic, phenology and ageing of arborescent Dracaena
sp. have been implemented.
The age of fabulous dragon trees is thus still generally
clouded in overestimation by Humboldt (1814) who
hypothesized that a huge Dracaena draco with 15 m stem
girth from Orotava, Tenerife was several thousand years
old. A more probable average figure of up to 700 years
old was suggested by Bystrm (1960). However, later
observations by Symon (1974) and Magdefrau (1975)
brought both a precise record of the age of cultivated trees
and also the first clues on determination of ageing.
Nevertheless, no authentic study on ageing of natural
populations of Dracaena tree species has been published.
The dragon tree group provides an example of two
major biogeographical disjunctions between East and
West Africa, being of Tethyan origin according to fossil
and paleoclimatic data. Hooker (1878) first proposed that
the dragon tree, together with other species of the
Macaronesian laurel forest, is probably a relic of an old
vegetation which once existed in northwestern Africa.
Axelrod (1975) proposed that subtropical elements, such
as Dracaena and Sideroxylon, found refuge in eastern and
western Africa, as a consequence of the desertification of
the Sahara in the late Oligocene (Quzel 1978). However,
it is also clear that Dracaena sp. stretched throughout the
Mediterranean region into southern Russia (Gwyne 1966).
According to pollen analyses from the Neogene, two
extinct Dracaena species have been identified. Pollen of
the first species, D. saportae Van Campo and Sivak,
comes from Bohemia whilst the second, D. guinetii Van
Campo and Sivak, was found in Tunez (Van Campo and
Sivak 1976). Current species of the dragon tree group are,
therefore, only a depleted and relic representation of the
Mio-Pliocene Laurasian subtropical flora. These species
supposedly thrived at the edge of closed forests. It is
likely that they occurred in sunny and exposed areas of
rocky slopes, cliffs and escarpments (Axelrod 1975).
The genus Dracaena comprises between 60 (Mabberley 1990) and 100 species (Bos in Kubitzki 1998). Recent
44
taxonomic ambiguity has caused its classification within
three families i.e. Agavaceae, Liliaceae and Dracaenaceae, the latter as a family arching over the former
confusion. Most of the Dracaena species grow as shrubs
or geophytes often having ornamental potential. There are
only six species having the growth habit of a tree. These
arborescent taxa of Dracaena can be found: (1) on
Socotra—D. cinnabari Balf., (2) in south-western Arabia—D. serrulata Baker, (3) in eastern Africa—D. ombet
Kotschy & Peyr, D. schizantha Baker, (4) on Macaronesian islands and in Morocco—D. draco L., in the latter
location distinguished as subsp. ajgal Benabid et Cuzin,
and (5) in Gran Canaria (Canary Islands) with the recently
described D. tamaranae A. Marrero, R.S. Almeida and M.
Gonzles-Martn.
All of these dragon tree species live in thermosclerophyllous plant communities of tropical-subtropical
regions which are rather xerophillous, sharing similar
ecological requirements with a rainfall range of 200–
500 mm and average temperatures of 18–20C (Marrero
et al. 1998). They are found between 10N (Somalia) and
33N (Madeira), and there is also some correlation
between latitude and altitude. Populations in Madeira
may be found at sea-level whilst those of Somalia never
occur below 1,400–1,800 m altitude (Bystrm 1960).
Socotra Island, once linked to peninsular Yemen,
probably comprises the woodiest part of what was once
called “Happy Arabia”. The island, with an area of
3,609 km2 (Aitken 2000) is situated on the African
continental shelf 225 km east of Cape Guardafui,
Somalia. At times it has been described with mythical
attributes such as Island of the Phoenix or The Abode of
Bliss, the latter probably owing to its biodiversity wealth
related to an exceptional 8% forest coverage that is
uncommon on mainland Arabia (PavliÐ 2000). The
prehistoric origin of local plants and their degree of
endemism reaching some 35% (Miller and Cope 1996)
place the island among the most environmentally significant places on Earth. Vegetation of the island, represented by several distinctive formations, derives from the
Tethyan flora (Bramwell and Richardson 1973) and its
unique character is the result of a long period of isolation;
separation from Africa supposedly occurred in the late
Cretaceous (Mies 1996).
Environmental conditions and quality throughout the
island are generally good and much better than those on
the major part of mainland Yemen. The island is
geologically characterized by the plutonic nucleus of the
granitic Hagghier Mountains located in its centre, and by
two similar crystalline massifs at the eastern and western
tips of the island, in places overlaid by sedimentary rocks
(Beydoun et al. 1970). Two limestone plateaus probably
correspond with similar layers on the nearby Somalian
peninsula (Kossmat 1907). The decomposition of granites
has led to the formation of rich red soils. The soils,
accumulated in the valleys and plains and even along the
less steep slopes, form the deepest and most fertile soil
deposits. An area within the confines of the limestone
plateau is dissected by gullies. Little pockets of fine, grey
clayey soil occur between rock crevices, but larger
shallow soil deposits have accumulated in depressions
where the gradient is slight (Popov 1957). A sufficient set
of climatic data is lacking; scarce information for a few
years in the 1940s and in 2000 (PavliÐ 2001) is all that is
available. The island generally falls into an arid tropical
zone, but with a climate tempered considerably by northeast and south-west monsoons, it is less torrid than that of
the adjacent mainland. Precipitation ranges between
200 mm for coastal plains and 1,000 mm (estimate
includes heavy horizontal rain-fall) for the highest
mountains (Popov 1957; Miller and Morris 2001).
From a biogeographical viewpoint, Socotra is evidently more closely linked with Africa than Arabia but there
are also interesting affinities with other island groups such
as the granitic Seychelles and even some remote Atlantic
islands (White 1983). Its three main biogeographical
zones can be distinguished as: (1) coastal plains, (2)
limestone plateaus, and (3) the Hagghier Mountains.
Species diversity projected through altitudinal vegetation zones is markedly influenced by orographical,
geological and pedological variability of the habitats.
The same is true on many of the Canary Islands (Sunding
1972). Some of the most dominant tree species often give
the landscape a rather bizarre or prehistoric appearance
(cf. Balfour 1888) with respect to their physiognomy and
structure. Prosperous shrubland and woodland can be
found mainly at higher altitudes and in sheltered valleys
dominated by xenomorphic growth forms as a result of
plant adaptation to local harsh conditions, i.e. desiccating
effects of strong winds and intensive solar radiation. The
most evident adaptations evolved to assist the plants’
survival include the swollen and silvery trunks of
Adenium obesum ssp. socotranum (Vierh.) Lav. and
Dendrosicyos socotranus Balf., and the distinctive umbrella or mushroom-shaped canopies of D. cinnabari
Balf. and Euphorbia arbuscula Balf. (Mies and Beyhl
1996).
Five to six principal vegetation tiers are consistently
distinguishable on Socotra (Miller et al. 1999; Buček
2001). Although prevalent physiognomic appearance is
related to shrubland category, woodland and even forest
categories are also present throughout all the tiers: (1)
coastal plains and low inland hills with prevailing
formations of open deciduous low shrubland (widespread
Croton socotranus Balf. and Jatropha unicostata Balf.),
(2) highlands with woodland character (variety of Commiphora sp. and Boswellia sp.), (3) limestone plateau with
semi-evergreen forests (optimal conditions for D. cinnabari), (4) granite mountains/higher part of limestone
plateau with evergreen forest and woodland (e.g. Euphorbia socotrana Balf., wild ancestor of pomegranate
Punica protopunica Balf.), and (5) alto-montane zone
with both evergreen forest (e.g. Euclea balfourii Balf.)
and dwarf cushion vegetation on the rocks.
Open forests of D. cinnabari can be found mainly in
the submontane zone. However, its common occurrence
ranges from 300 to 1,500 m alt., preferring limestonebased subsoil. This area is mainly covered by thickets of
45
Fig. 1 The last remnant of ’Tethys woodland’
Rhus thyrsiflora Balf., Cephalocroton socotranus Balf.,
Allophylus rhoidiphyllus Balf., Boswellia ameero Balf., B.
socotrana Balf., Jatropha unicostata Balf. and Croton
socotranus Balf. At higher altitudes, this type of thicket is
intermixed with Hypericum shrubland. D. cinnabari
contributes significantly to the biodiversity value of the
island (see Fig. 1).
The first inhabitants, represented by south-Semitic
nomadic tribes, probably came to the island some
3,000 years ago (Naumkin 1993). Over many centuries
they developed a relatively balanced land management
system, securing self-sufficiency in food for a scattered
population. As a matter of fact, grazing practice naturally
influenced plant communities and notably contributed to
the contemporary distribution and structure of tree
populations around the island, including endemic Commiphora spp., Boswellia spp., Dendrosicyos socotranus
and D. cinnabari.
Materials and methods
Two prolonged visits to the island in 2000 and 2001 occurred in
varied weather seasons in transition, February/March, October/
November and August/September. Attention paid to D. cinnabari
specimens throughout the island has brought together records on
phenology, habitat features, species composition, space and age
structure of D. cinnabari populations and details of indigenous
pastoral management influencing the guild. This species analysis
was conducted as a follow-up of earlier vegetation investigations on
Socotra Island undertaken by several scientific teams (Mies and
Beyhl 1996; Miller 1989–2001, unpublished data; Petroncinni and
Ceccolini, unpublished data). With a view to elucidate growth pace
and ageing of Dracaena, supplementary microscopic analyses were
also carried out (Adolt 2001). In order to illustrate regeneration
processes of D. cinnabari under natural conditions and to obtain
reliable information on germinative capacity, germination experiments were undertaken, both in the field and in the glasshouse
(PavliÐ 2001).
Fig. 2 ‘Sausage-shaped’ branch sections or orders of branches
Analyses of the age structure of natural populations of D. cinnabari
An indirect method based on architectural age and statistical
analyses were applied to analyze age structure of D. cinnabari
population on Socotra. Accordingly, information was collected on
growth features and dimensions of particular specimens of D.
cinnabari near Firmihin (three localities) 12280 2800 N, 54000 5400 E,
altitude 440 m; and Hamadero 12350 1200 N, 54180 900 E, 328 m
altitude. A randomly situated set of 50 neighboring specimens at all
four localities was recorded, including the following parameters:
(1) number of swollen sections on a randomly selected peripheral
branch, (2) height of tree (m), (3) girth at breast height (cm), (4)
length of a stem (m), (5) crown dimensions (m) and (6) spacing of
D. cinnabari trees (m) in order to obtain spatial structure
characteristics of the localities under study. Linear measures were
taken with a tape, tree heights with a hypsometer/clinometer SilvaClinomaster. Data were processed using exploratory data analysis
(EDA), theory of estimation and probability distribution. Details of
mensurational data of trees are not part of this paper (Adolt 2001;
PavliÐ 2001). At Firmihin additional sampling was also carried out,
consisting of 22 dead dragon trees (most already uprooted), to
obtain a reference set of the probable age of overmature trees.
For each analyzed dragon tree at the given locality, sampling age
was calculated according to Eq. 2 (see below). Age sampling obtained
in this way was then analyzed. It has been found that it is possible to
determine normal distribution in all samplings. For testing normality
the following tests were used: diagrams of normal probability,
diagrams of empirical density of probability, test of the combination
of sampling skewness and kurtosis and Kolmogorov-Smirnov test
(Meloun and Militk 1998). According to the determined parameters
of distribution (mean values and standard deviations), diagrams of
probability densities and diagrams of distribution functions of dragon
tree age in particular populations were constructed.
An indirect method proposed in this paper for age structure
analysis is related to the search for a model reflecting relationships
between the number of flowering periods and the actual age of a
specimen. The relationship can be well defined through specific
branching of arborescent Dracaenas characterized by markedly
swollen branches segregated individually by narrowed sections or
‘sausage-shaped’ sections (see Fig. 2). These obviously relate to the
growth rhythm of a tree and correspond to one period of flowering.
46
Fig. 3 The model of a relation between the number of growth tops
of the crown periphery and the number of orders of peripheral
branches. Lines Yp+, Yp- depict a region where all linear models
without an absolute term will occur with 95% probability. Lines
Y+, Y- depict a region where all experimentally determined values
of the number of growth tops of the crown periphery in dependence
on the number of orders of peripheral branches will occur with 95%
probability
This branching peculiarity was first described by Symon (1974)
and Magdefrau (1975) from observations of cultivated specimens
of D. draco. However, it was not statistically utilized for any
generalization of age determination. The presented indirect method
of age determination is based on the number of orders of peripheral
branches of the crown distinguishable according to their characteristic ‘sausage-shaped’ form. Growth of the tree apex is
terminated after flowering followed by branching which is often
monochasial in the higher order of branches. In older branches
(branches of several first orders), distinguishing the part corresponding to one flowering period may be more difficult due to
secondary swelling of the branch which can result in suppression of
the typical ‘sausage-shaped’ form of a branch section. From the
vantage point of the top, after climbing a tree, it is possible to see
very distinct, particular orders even in older branches—a trace after
a peduncle which remains noticeable even after considerable
swelling of the branch. For the purpose of developing a theory of
age estimation, 30 D. cinnabari trees in the Firmihin area were
analyzed. The number of ‘sausage-shaped’ sections (orders of
peripheral branches of the crown) was determined (round mean
value from several randomly selected branches), as well as the
number of crown tops (i.e. individual ‘dragon heads’) on the crown
periphery, the number of fruiting tops and the number of flowers on
the crown periphery. Based on the data obtained, (1) a model was
developed of the dependence between the number of crown tops on
the crown periphery and the number of orders of peripheral
branches, and (2) a relationship was derived between the number of
fruiting tops of the crown periphery and the total number of tops of
the crown periphery. The following considerations and preconditions were dealt with:
– Average number of tops on crown periphery
– Average number of tops flowering yearly
– Average time necessary for increasing the number of branch
orders along the whole crown periphery by just one order.
– In general, it is not possible to suppose that trees are in flower in
all crown tops simultaneously (in the same year); evidently only
part of all tops is in flower.
– There is a significant correlation between the number of tops on
the crown periphery and the number of orders of peripheral
branches.
– The number of branch orders of the whole crown periphery is
increased in a given age by one (within a period which is equal
to the ratio of the mean number of tops and the mean number of
yearly flowering tops).
– The ratio is not changed if only the crown periphery is
considered.
– Flowers can appear in various seasons of the year and, therefore, it
is more suitable to use fruit panicles which persist on a tree for a
longer time than flower panicles but not longer than 1 year.
For the given number of orders of peripheral branches it is
possible, according to established models, to find:
In this way, it is possible to estimate the age of a D. cinnabari
crown. For the final estimate of tree age, it is still necessary to find
the tree age at the time of first flowering (in the majority of cases,
also of first branching). The estimate of age rate made on the basis
of several non-ramified trees of known age served this purpose. An
indigenous “gardener” estimated the age of one D. cinnabari
specimen he was cultivating for ornamental purposes. However,
local understanding of the passage of years is not very exact. Based
on the information provided, it was supposed that in the time before
the first flowering, the D. cinnabari stem showed a height growth
corresponding to about 10 cm/year. According to the known stem
length of the analyzed tree it is then possible to estimate the age
when the first flowering and branching occurred. The sum of tree
age in the time of first flowering and the age of the tree’s crown
gives the estimate of its total age. The estimate of tree age can then
be expressed according to the following relation:
Zx
f ð xÞ
lstem
V¼
dx þ
ð1Þ
g½f ðxÞ
dyear
0
where V is the tree age,
Rx
is the definite integral from zero to x, f(x)
0
is the function of the relation between the number of tops on the
crown periphery and the number of orders of peripheral branches,
g[f(x)] is the function of the relation between the number of
annually fruiting tops of the crown periphery and the total number
of tops of the crown periphery, Istem is stem length, and dyear is the
mean annual length increment of a stem.
Based on the above mentioned conclusions, three models were
constructed aimed at identifying proper relations for age assessment
of D. cinnabari .
Model 1: a model of the relation between the number
of growth tops of the crown periphery and the number of orders
of peripheral branches
There is a statistically significant linear dependence (probability of
invalidity of the thesis is <1%) between the variables. The
correlation coefficient value amounts to 0.716 (95% confidence
interval 0.479–0.855). The significance test was carried out using
Ruben transformation (Meloun and Militk 1998). The linear
model was accepted, the model diagram and its parameters are
47
Fig. 4 The model of a relation between the number of annually
fruiting growth tops of the crown periphery and the number of all
growth tops of the crown periphery. Lines Yp+, Yp- depict a region
where all linear models without an absolute term will occur with
95% probability. Lines Y+, Y- depict a region where all experimentally determined values of the number of growth tops of the
crown periphery in dependence on the number of orders of
peripheral branches will occur with 95% probability
given in Fig. 3. The model represents an important improvement in
prediction as opposed to the use of a simple arithmetic mean of the
interpreted variable which was demonstrated by a test based on the
analysis of variance. The linear model is acceptable for the
description of dependence, verified by a test according to Utts
(1982). The model does not include an absolute term. The term was
not taken into consideration because in a previous calculation, it
was shown to be insignificant. The absence of an absolute term is
also in agreement with the idea that with zero number of orders of
peripheral branches, the number of growth tops of a crown also has
to be zero.
A relation1k in the formula corresponds to the estimate of the
average time (expressed in years) necessary for increasing the
number of branch orders of the whole crown periphery by just one
order.
Model 2: a model of the relation between the number of annually
fruiting tops of the crown periphery and the total number of growth
tops of the crown periphery
A linear correlation coefficient between the values of the variables
amounts to 0.534 (95% confidence interval 0.215–0.750). The
correlation coefficient is statistically significant; probability of
error of the thesis does not exceed 1%. The model and its
parameters are given in Fig. 4. Based on the test according to Utts
(1982) the linear model was found to be acceptable for the
dependence description (Meloun and Militk 1998). The regression
function is statistically significant (its use for prediction is more
suitable than the use of arithmetical mean of a dependent variable).
The model does not include an absolute term because in a previous
calculation it was shown to be insignificant. Similar to the model
mentioned above, there is a logical substantiation of the absolute
term insignificance. It is based on the fact that with zero number of
tops on the crown periphery, the number of yielding tops on the
crown periphery also has to be zero.
Model 3: a model of the relation between tree age and the number
of orders of peripheral branches and stem length
Since both previous models are linear and do not include an
absolute term, the general relation Eq. 1 for the calculation of tree
age changes to a simpler relation:
V¼
1
lstem
Xþ
dyear
k
ð2Þ
whereV is the tree age, k is the linear term of the dependence model
of the number of annually fruiting tops of the crown periphery on
the total number of tops of the crown periphery (a value of
0.053388), X is the number of orders of peripheral branches, Istem is
the stem length, and dyear is the mean annual length increment of a
stem.
Results and discussion
Peculiarities of growth
In many respects Dracaenaceae are implicitly exceptional
among monocotyledonous plants. One of the remarkable
features is their secondary growth. There are even curious
growth zones on parts of the stem or branch of D.
cinnabari (Adolt 2001) resembling growth rings of a
temperate zone. They are, however, of no use for age
estimations due to the probability of being caused by
specific crossing of vascular bundles (Razdorskij 1954;
Tomlinson and Zimmermann 1969). This secondary
tissue consists of scattered vascular strands, and is quite
unlike that of dicotyledonous trees (Hall et al. 1978).
However, there is a close structural and developmental
relationship between primary and secondary vascular
bundles, according to Tomlinson and Zimmermann
(1970), revealing closeness of developmental steps from
primary to secondary growth.
Another distinctive feature of the arborescent Dracaena group is its growth habit called ‘Dracoid habitus’ (Bos
1984; Lsch et al. 1990) or coined as Leeuwenberg’s
model (Hall et al. 1978). It can be found in plants of
several families, and was indeed present in extinct plants
such as Lepidodendron sp., Bothrodendron sp. or Sigillaria sp. of the Carboniferous period (Stewart and
Rothwell 1993). D. cinnabari bears its leaves exclusively
at the end of the youngest branches arranged in clusters
(‘rosette trees’) like similar ‘dracoid’ plants. The oldest
leaves seem to be shed en bloc every 3–4 years with
simultaneous maturing of new leaves sensu ‘leaf-exchanging-type with periodic-growth’ (Longman and Jenk
1987), influenced by incessant growth of the branch at its
tip. Interestingly, some ‘dracoid’ species shed all their
48
leaves each year at the beginning of the dry period. In
other words, ‘dracoid habitus’ denotes a special way of
growth and ramification the details of which are further
exemplified by the example of D. cinnabari.
There always arise more or less symmetrical branches
from the upper end of a trunk, being somewhat inflected,
rarely rectilinear, but branching again at their ends, to
form the second, third, etc. orders (see details of the
crown in Fig. 2). Branching normally occurs only when
growth of the terminal bud is stopped, either by flowering
or by a traumatic event. However, branches often develop
only one single secondary branch at their tip, after
cessation of growth, so that they look as if they had not
ramified (Beyhl 1996). Large erect terminal panicles of
flowers expand from the terminal bud of the youngest
branches and, after it has matured, it is pushed aside as
one or more axillary buds develop in the axils of the
leaves immediately below the inflorescence (Symon
1974). This new shoot replaces the parental axis; growth
of each swollen branch unit always ends after blooming
(Bos in Kubitzki 1998). One or more buds from the
nearby leaf axils will then be activated to grow and to
form a new branch, although occasionally in a pseudodichotomous way. Branches are, therefore, characterized
by pronounced swollen units segregated individually one
from another by a narrowed joint obviously related to tree
growth rhythm over the course of time. Each axis is,
therefore, a sympodium consisting of many successive
growth units (Symon 1974). The sympodium appears
articulate both from the scars of the dried inflorescence
stalks and from a swelling which marks each joint
(Zimmerman and Tomlinson 1969). These slightly
swollen units, marked by constrictions, are referred by,
e.g., Bystrm (1960) as ‘generations of branches’. Such a
ramification system of branches of several orders subsequently yields a very regularly shaped, umbrella-like
crown (Beyhl 1996). These swollen parts or ‘sausageshaped’ sections have incomparable value for both age
assessment of individual trees and whole Dracaena sp.
populations in natural stands, since wood anatomical
structure, classical dimensional characteristics as girth at
breast height (g.b.h.), or tree height do not provide
credible clues.
In spite of that, D. cinnabari flowering periods seem to
differ notably on Socotra from locality to locality (even
ever-flowering is probable), D. cinnabari blooming and
subsequent fruiting always take only a certain percentage
of crown ‘dragon heads’ during “mankind’s time marker”
(1 year). Fruiting assessment described in model 2 of the
methodology above indicates percentage of fruiting heads
as low as 5% per year (cf. Symon 1974). Notwithstanding
some difficulties, a model can be deduced for crown
fruiting in the course of time that brings important clues
for age estimation of either trees or D. cinnabari
populations (vide infra).
The fruits are globose, fleshy berries changing from
green to black as they ripen and becoming orange-red in
the last stage. Berries bear 1–3 seeds that are then eaten
by birds (e.g. Onychognatus sp.) which function as
dispersal agents. Although berries of both species have
nearly the same size (Beyhl 1996), D. cinnabari seeds are
considerably smaller than those of D. draco. There are
also differences in the number of seeds inside a berry.
According to the present study, the quantity of 2- to-3seeded berries of D. cinnabari reaches as high as 38.3%,
as compared to the berries of D. draco that contain only
one seed with 97.4% probability. D. cinnabari has got in
95% of cases a seed diameter between 4.13 and 4.98 mm.
On the other hand, D. draco seeds have diameters from
6.60 mm to 8.81 mm with the same probability. Weight of
the seeds also differs significantly—i.e. 1,000 seeds of D.
cinnabari weigh only 68 g whereas 1,000 seeds of D.
draco weigh 335 g (Adolt 2001).
Age structure of D. cinnabari populations
Knowledge of the tree population age structure is a key
factor for determining the procedure and urgency of their
early and effective conservation. However, it is a very
time-consuming task to determine the age of trees,
whether monocotyledonous or dicotyledonous, by their
direct measurement over time. In spite of this, precise
yield tables for main production tree species have been
developed through generations of foresters in temperate
commercial forests. Such a direct method also brought the
first clues about the ageing of D. draco (Symon 1974;
Magdefrau 1975). Nevertheless, indirect methods based
on architectural age and statistical analyses (Hall et al.
1978) seem to be more suitable for age determination of
monocotyledonous trees. In that indirect way, age structure estimation of D. cinnabari population on Socotra was
carried out.
Based on the above-mentioned model 3 of the
dependence of D. cinnabari tree age on the number of
orders of peripheral branches, the problematic nature of
determining the exact age of individual trees is evident.
The problem is related to the difficulty of estimating the
age of a tree at the first flowering and the total crown age.
The assessment of crown age is limited by a potential
error in determining the coefficient k of a regression
model (see Eq. 2). An error in the estimate of coefficient k
shows a zero mean value and with 95% probability its
absolute value does not exceed a limit of 0.0196. The
average time necessary for increasing the number of
orders of peripheral branches of the whole crown by just
1
1
one order shows limits kþ0:0196
, i.e. 13.7 years and k0:0196
,
1
i.e. 29.6 years and a mean value equal to k, i.e. 18.7 years.
If a tree of D. cinnabari is considered with n orders on
peripheral branches then the actual age of the crown (with
95% probability) will be in the interval n 13.7 years to n
29.6 years and a mean value will most probably be n
18.7 years. With respect to the small range of data, it is
not possible to report in detail about errors of age estimate
of the first flowering. It is possible to suppose only that
the errors affect tree age determination less significantly
than those in estimation of crown age.
49
Fig. 5 Diagram of probability
density for age in particular
populations under study. Mean
age of particular populations on
the age axis corresponds to
curve tops
Fig. 6 Diagram of distribution
functions of age for selected
populations of Dracaena cinnabari. Probability 0.5 corresponds to the arithmetical mean
of age on the age axis for the
given population
The range of probable age of one ‘sausage-shaped’
section of D. cinnabari determined by our team thus
reaches higher values, i.e. 13.7–29.6 years, than findings
of Symon (1974). His findings concerning probable age of
one ‘sausage-shaped’ section of D. draco indicate for (1)
Adelaide Zoo 11 orders of constrictions, each being 9–
10 years old; and for (2) Victoria Park Garden 5–6 orders,
being 11–14 years old. However, our data are close to
Magdefrau’s (1975) direct findings from Tenerife where
he stated that age of one ‘sausage-shaped’ section ranged
from 11–17 to 13–16 years. Although the authors
mentioned above used the direct method of age estimation
they presumably have not used statistics to estimate error
probability of their findings.
Comparisons of D. cinnabari populations on Socotra
from the viewpoint of age, implemented by means of a
single factor ANOVA with fixed effects (Meloun and
Militk 1998), brought a finding that at least two
populations significantly differed in their average age
probability, with the error of predication being
2.409E13. The Dunnett test procedure of a multiple
comparison (Meloun and Militk 1998) was then applied.
As a comparative group, the sample of dead dragon trees
from the Firmihin area was used. Figure 5 describes the
most probable age structure of analyzed natural populations of D. cinnabari on Socotra.
According to the position of curves for particular
populations with respect to the curve of dead D. cinnabari
trees (Fig. 6), we can deduce the amount of time needed,
on average, for a given population to reach the value of
average age of D. cinnabari death, i.e., to the stage of
advanced decay (under conditions of zero regeneration of
50
Table 1 Mean age of DC populations according to localities and estimate of probable time scope to their disintegration stage
Population
Firmihin 3
Firmihin 4
Firmihin 5
Hamadero 1
Mean age of DC
population (years)
200
257
294
228
Estimation of a probable time scope to the desintegration stage of DC population on Socotra
Island (years)
The most probable estimation k
Lower 95% estimation k
Upper 95% estimation k
144
87
50
116
97
56
30
80
230
138
77
178
Fig. 7 Dependence of the
length of a ‘sausage-shaped’
section, i.e. order of branch, on
its hierarchical position in tree
crown order
population). For similar analyses, however, it would be
necessary to model the development of populations on the
basis of known age distributions in populations, the
course of D. cinnabari decline, seed germination and
young plant mortality. The following table includes an
overview of the expected stage of intensive disintegration
at particular localities under analysis (Table 1).
According to the results of the Dunett test of multiple
comparisons, the mean age of D. cinnabari trees at all
localities is lower than that of dead D. cinnabari;
probability of invalidity of the thesis is lower than 1%.
The procedure described for age determination is
based on preconditions that can be further complemented
and/or specified. Most important is the fact that fruits
persist for about 1 year on floral panicles so that
according to the number of panicles, it is possible to
determine the number of flowering tops of the crown
periphery in a given year. This observation needs to be
supported by further field investigations. In case of
finding new facts, it would be possible to specify the most
suitable time for obtaining data for the model construction. In any case, it will be useful to analyze a greater
number of trees for the purpose of obtaining more precise
estimates of present models (narrowing the confidence
interval of coefficient k) or for possibly finding more
suitable models (non-linear) describing the dependencies
mentioned above.
The model of the dependence of annually fruiting tops
of the crown periphery on the total number of crown tops
could be non-linear. Non-linear dependence was also
found between branch order and its length. The model is
depicted by the diagram below (see Fig. 7). The model is
statistically significant, being constructed on the basis of
crown analysis of one D. cinnabari tree. A decrease in the
length of a ‘sausage-shaped’ section towards higher
orders of branches is visually evident in all D. cinnabari
trees. There is, however, a question as to what extent the
dependence is related to growth dynamics and to what
extent to changes in flowering frequency of particular
tops in relation to age (branch order). If the dependence is
a result of growth dynamics alone, the model of the
number of annually flowering tops of the crown periphery, as related to the total number of tops of the crown
periphery, is linear. It is, however, more probable that the
dependence is affected by both factors—growth dynamics
and changes in flowering frequency of particular tops
over time.
The potential of natural regeneration and further seral
development of D. cinnabari populations
In order to verify the potential of natural regeneration of
D. cinnabari forests a controlled germination experiment
was performed in a glasshouse at the Mendel University
of Agriculture and Forestry (MUAF), Brno. Results
reflect generally low germination capacity of D. cinnabari after direct sowing on soil, after stimulation by H3CCOOH; only 5% germination was reached. However, the
capacity increased to 22% using 1% H2O2 solution.
51
Meanwhile, the experiment indicates that germination
capacity of D. draco seed (used for comparison), under
optimum conditions, can reach at least 77% (Adolt 2001).
Even though the seed experiment in the glasshouse at
MUAF was finished in April 2001 (remaining seeds were
removed from blocks and discarded), pots with germinated seeds were maintained in three other glasshouses as
a control. These glasshouses did not have guaranteed
microclimatic conditions, but onward gradual germination was recorded (even reaching a surprising 20%) as
temperature increased through the spring, summer and
early autumn in 2002. It therefore seems to support the
hypothesis of higher temperature as a trigger for germination of D. cinnabari.
There are, however, significant differences between
the present results and the findings of Beyhl (1996) who
mentions D. cinnabari germination capacity of as high as
35%. Such a divergence can naturally be caused by many
factors, ranging from germination energy variation among
phenotypes to different glasshouse conditions or effects of
mycorrhizal requirements.
Averaging the number of branches calculated per tree
(i.e., 1,056 branches per tree) and extrapolating data on
the number of berries per yielding panicle, i.e. 580 berries
on average (nine panicles counted), the interesting
probable number of 611,000 seeds is achieved during
the life span of an average tree. Comparing the data with
the above mentioned ideal germination capacity of 22%,
another interesting figure of about 134,000 potentially
germinated seeds per tree is computed.
Seedling vitality in a glasshouse tends to be quite
satisfactory as mortality rate is only about 10%. However,
field observations indicate that seedling mortality rate,
under climatic conditions of Socotra Island, rises up to
100% if a subsequent favorable period of rainfall does not
come (also Miller and Morris, personal communication).
In conformity with that, even a field experiment with 200
seeds sown inside a game-proof fence (PavliÐ 2001) did
not provide a single germinated seedling after 1 year.
The facts mentioned above bring some doubts about
the prevalent assumption of strong browsing effects of
livestock on any tree species regeneration (except
poisonous plants), especially the influence of ubiquitous
goats whose number reaches about 30,000 (Clarke and
Dutton 2000). The presence of livestock is related to the
first human habitation dating back some 3,000 years
(Naumkin 1993) and revision of their land management
practices confirmed prevailing sustainability within contextual birth-rate. Although leaves of D. cinnabari are in
general unpalatable to cows and goats according to
interviews with indigenous community members, our own
field observations indicate that goats nevertheless do feed
on fresh leaves of windfall trees. Interestingly, even
though leaves are not considered palatable pasturage by
herders, they themselves use a limited quantity of
collected berries to feed cows in times of drought.
Although certain age uniformity of most of the D.
cinnabari stands is remarkable, there is an interesting
exception related to the occurrence of the youngest trees
and seedlings on steep and extreme cliffs (at all altitudes
of D. cinnabari occurrence). This small-scale success of
D. cinnabari reproductive strategy can be consequently
related either to a more favorable moisture regime for
seed germination inside stone pockets of those cliffs or to
habitat inaccessibility to browsing animals.
The umbrella-shaped canopy of D. cinnabari also
apparently helps seedlings to thrive better in its shadow
than in full sun. This umbrella type of crown also
functions as an adaptation to intercept humidity from
mist, horizontal rainfall and dew, later falling down from
the branches. This is of course the way to increase soil
water potential below the crown. Moreover, the canopy
shadow reduces soil water evaporation to values lower
than elsewhere in the environment (Mies and Beyhl
1996). This favorable effect of the ‘crown type’ for
contingent germination could presumably be the reason
why many D. cinnabari trees grow close together,
forming an interconnected crown unit, i.e., younger
intermediate specimen thriving under older one growing
into crown above, gradually taking over the crown space,
forming finally either coexisting or competing twins.
At the same time, it is also difficult to expect that
global climatic changes would be the only factor negatively influencing D. cinnabari regeneration, taking into
account the case of the Diksam plateau (tableland below
Hagghier) where, in the past, one of the largest D.
cinnabari forests supposedly existed. At present, this area
is in a stage of deep disintegration with too few scattered
patriarch trees (perhaps up to 350 years old). Woodland of
this area obviously started to decay more than 100 years
ago.
There is, accordingly, stronger evidence for the
hypothesis of a sudden mass regeneration in favorable
short periods interrupting the standard long-lasting semiarid weather pattern. However, it is still difficult to judge
if this sudden mass recovery wave is caused by favorable
climatic conditions or follows a decline of livestock
population, i.e., recurrent animal-unfavorable deficiency
of water and feed. This D. cinnabari reproduction strategy
is slightly similar to the forest reproduction pace of
natural monocoenotic overmature spruce forest of a
boreal zone which regenerates over large areas following
a zero event (Oldeman 1983) caused by wind, fire, or
bark-beetle attack. Such a presumption of the species
regeneration course is, moreover, supported by the very
uniform age structure of D. cinnabari populations on most
of the optimum habitats on Socotra.
Based on statistical analyses as well as on direct field
observations, D. cinnabari populations on Socotra, however, do not regenerate to a great extent and their age
structure generally indicates overmaturity. According to
analysis of dead trees, it is evident that D. cinnabari
populations of Firmihin die on average after reaching 17
orders of peripheral branches. Therefore, it is possible to
suppose that a large part of the world’s most extensive
‘Dracaena woodland’ complex in area of Firmihin can be
assessed from the viewpoint of age as a population in the
area of locality Firmihin 5. It means that, with 95%
52
probability, stands here will be in the stage of intensive
disintegration within 30–77 years. Such an alarming pace
of likely Dracaena forest disappearance caused by the
pressure of civilization already has some parallel with the
islands of Tenerife, La Palma or Gran Canaria (Canary
Islands) where forests of D. draco and D. tamaranae have
not survived except for a few scattered trees currently
located mainly on the most inaccessible cliffs.
Therefore, if the D. cinnabari woodland is to be
preserved at its present extent, it is most urgent to start
monitoring its natural regeneration, since we may already
be at the 11th hour. Also, there is still a strong hope that
the rich and unique biodiversity of Socotra—unrivalled in
the Indian Ocean and in the Arabian region—will be
maintained through the successful conservation and wise
developmental efforts of national and international agencies.
Acknowledgements This study was facilitated mainly by the kind
support of the Czech Grant Agency GACR under the POSTDOC
project No.526/99/P044. We greatly appreciate the kind logistical
backing of the Environmental Protection Council of Yemen, and
particularly wish to thank its deputy-director, Dr. Al-Guneid. We
also gratefully acknowledge the scientific advice of our university
colleagues: Dr. A. Bucek for enthusiastic support during all stages
of the work, Dr. M. Martinkova for her suggestions concerning
plant anatomy analysis, Drs. P. Jelinek and H. Habrova for their
helpful company in the field. Dr. T. Miller from RBGE deserves
our gratitude for sharing his long-term observations. Last but not
least, full credit is given to Mohammad “Al Keybn” for his
guidance through the mysterious “Island Abode of Bliss”.
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